Internet Engineering Task Force (IETF) E. Rescorla
Request for Comments: 5746 RTFM, Inc.
Updates: 5246, 4366, 4347, 4346, 2246 M. Ray
Category: Standards Track S. Dispensa
ISSN: 2070-1721 PhoneFactor
N. Oskov
Microsoft
February 2010
Transport Layer Security (TLS) Renegotiation Indication Extension
Abstract
Secure Socket Layer (SSL) and Transport Layer Security (TLS)
renegotiation are vulnerable to an attack in which the attacker forms
a TLS connection with the target server, injects content of his
choice, and then splices in a new TLS connection from a client. The
server treats the client's initial TLS handshake as a renegotiation
and thus believes that the initial data transmitted by the attacker
is from the same entity as the subsequent client data. This
specification defines a TLS extension to cryptographically tie
renegotiations to the TLS connections they are being performed over,
thus preventing this attack.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5746.
Rescorla, et al. Standards Track [Page 1]

RFC 5746 TLS Renegotiation Extension February 20101. Introduction
TLS [RFC5246] allows either the client or the server to initiate
renegotiation -- a new handshake that establishes new cryptographic
parameters. Unfortunately, although the new handshake is carried out
using the cryptographic parameters established by the original
handshake, there is no cryptographic binding between the two. This
creates the opportunity for an attack in which the attacker who can
intercept a client's transport layer connection can inject traffic of
his own as a prefix to the client's interaction with the server. One
form of this attack [Ray09] proceeds as shown below:
Client Attacker Server
------ ------- ------
<----------- Handshake ---------->
<======= Initial Traffic ========>
<-------------------------- Handshake ============================>
<======================== Client Traffic ==========================>
To start the attack, the attacker forms a TLS connection to the
server (perhaps in response to an initial intercepted connection from
the client). He then sends any traffic of his choice to the server.
This may involve multiple requests and responses at the application
layer, or may simply be a partial application layer request intended
to prefix the client's data. This traffic is shown with == to
indicate it is encrypted. He then allows the client's TLS handshake
to proceed with the server. The handshake is in the clear to the
attacker but encrypted over the attacker's TLS connection to the
server. Once the handshake has completed, the client communicates
with the server over the newly established security parameters with
the server. The attacker cannot read this traffic, but the server
believes that the initial traffic to and from the attacker is the
same as that to and from the client.
If certificate-based client authentication is used, the server will
see a stream of bytes where the initial bytes are protected but
unauthenticated by TLS and subsequent bytes are authenticated by TLS
and bound to the client's certificate. In some protocols (notably
HTTPS), no distinction is made between pre- and post-authentication
stages and the bytes are handled uniformly, resulting in the server
believing that the initial traffic corresponds to the authenticated
client identity. Even without certificate-based authentication, a
variety of attacks may be possible in which the attacker convinces
the server to accept data from it as data from the client. For
instance, if HTTPS [RFC2818] is in use with HTTP cookies [RFC2965],
the attacker may be able to generate a request of his choice
validated by the client's cookie.
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RFC 5746 TLS Renegotiation Extension February 2010
Some protocols -- such as IMAP or SMTP -- have more explicit
transitions between authenticated and unauthenticated phases and
require that the protocol state machine be partly or fully reset at
such transitions. If strictly followed, these rules may limit the
effect of attacks. Unfortunately, there is no requirement for state
machine resets at TLS renegotiation, and thus there is still a
potential window of vulnerability, for instance, by prefixing a
command that writes to an area visible by the attacker with a command
by the client that includes his password, thus making the client's
password visible to the attacker (note that this precise attack does
not work with challenge-response authentication schemes, but other
attacks may be possible). Similar attacks are available with SMTP,
and in fact do not necessarily require the attacker to have an
account on the target server.
It is important to note that in both cases these attacks are possible
because the client sends unsolicited authentication information
without requiring any specific data from the server over the TLS
connection. Protocols that require a round trip to the server over
TLS before the client sends sensitive information are likely to be
less vulnerable.
These attacks can be prevented by cryptographically binding
renegotiation handshakes to the enclosing TLS cryptographic
parameters, thus allowing the server to differentiate renegotiation
from initial negotiation, as well as preventing renegotiations from
being spliced in between connections. An attempt by an attacker to
inject himself as described above will result in a mismatch of the
cryptographic binding and can thus be detected. The data used in the
extension is similar to, but not the same as, the data used in the
tls-unique and/or tls-unique-for-telnet channel bindings described in
[TLS-CHANNEL-BINDINGS]; however, this extension is not a general-
purpose RFC 5056 [RFC5056] channel binding facility.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Secure Renegotiation Definition3.1. Additional Connection State
Both client and server need to store three additional values for each
TLS connection state (see RFC 5246, Section 6.1). Note that these
values are specific to connection (not a TLS session cache entry).
Rescorla, et al. Standards Track [Page 4]

RFC 5746 TLS Renegotiation Extension February 2010
o a "secure_renegotiation" flag, indicating whether secure
renegotiation is in use for this connection.
o "client_verify_data": the verify_data from the Finished message
sent by the client on the immediately previous handshake. For
currently defined TLS versions and cipher suites, this will be a
12-byte value; for SSLv3, this will be a 36-byte value.
o "server_verify_data": the verify_data from the Finished message
sent by the server on the immediately previous handshake.
3.2. Extension Definition
This document defines a new TLS extension, "renegotiation_info" (with
extension type 0xff01), which contains a cryptographic binding to the
enclosing TLS connection (if any) for which the renegotiation is
being performed. The "extension data" field of this extension
contains a "RenegotiationInfo" structure:
struct {
opaque renegotiated_connection<0..255>;
} RenegotiationInfo;
The contents of this extension are specified as follows.
o If this is the initial handshake for a connection, then the
"renegotiated_connection" field is of zero length in both the
ClientHello and the ServerHello. Thus, the entire encoding of the
extension is ff 01 00 01 00. The first two octets represent the
extension type, the third and fourth octets the length of the
extension itself, and the final octet the zero length byte for the
"renegotiated_connection" field.
o For ClientHellos that are renegotiating, this field contains the
"client_verify_data" specified in Section 3.1.
o For ServerHellos that are renegotiating, this field contains the
concatenation of client_verify_data and server_verify_data. For
current versions of TLS, this will be a 24-byte value (for SSLv3,
it will be a 72-byte value).
This extension also can be used with Datagram TLS (DTLS) [RFC4347].
Although, for editorial simplicity, this document refers to TLS, all
requirements in this document apply equally to DTLS.
Rescorla, et al. Standards Track [Page 5]

RFC 5746 TLS Renegotiation Extension February 20103.3. Renegotiation Protection Request Signaling Cipher Suite Value
Both the SSLv3 and TLS 1.0/TLS 1.1 specifications require
implementations to ignore data following the ClientHello (i.e.,
extensions) if they do not understand it. However, some SSLv3 and
TLS 1.0 implementations incorrectly fail the handshake in such a
case. This means that clients that offer the "renegotiation_info"
extension may encounter handshake failures. In order to enhance
compatibility with such servers, this document defines a second
signaling mechanism via a special Signaling Cipher Suite Value (SCSV)
"TLS_EMPTY_RENEGOTIATION_INFO_SCSV", with code point {0x00, 0xFF}.
This SCSV is not a true cipher suite (it does not correspond to any
valid set of algorithms) and cannot be negotiated. Instead, it has
the same semantics as an empty "renegotiation_info" extension, as
described in the following sections. Because SSLv3 and TLS
implementations reliably ignore unknown cipher suites, the SCSV may
be safely sent to any server. The SCSV can also be included in the
SSLv2 backward compatible CLIENT-HELLO (see Appendix E.2 of
[RFC5246]).
Note: a minimal client that does not support renegotiation at all
can simply use the SCSV in all initial handshakes. The rules in the
following sections will cause any compliant server to abort the
handshake when it sees an apparent attempt at renegotiation by such a
client.
3.4. Client Behavior: Initial Handshake
Note that this section and Section 3.5 apply to both full handshakes
and session resumption handshakes.
o The client MUST include either an empty "renegotiation_info"
extension, or the TLS_EMPTY_RENEGOTIATION_INFO_SCSV signaling
cipher suite value in the ClientHello. Including both is NOT
RECOMMENDED.
o When a ServerHello is received, the client MUST check if it
includes the "renegotiation_info" extension:
* If the extension is not present, the server does not support
secure renegotiation; set secure_renegotiation flag to FALSE.
In this case, some clients may want to terminate the handshake
instead of continuing; see Section 4.1 for discussion.
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RFC 5746 TLS Renegotiation Extension February 2010
* If the extension is present, set the secure_renegotiation flag
to TRUE. The client MUST then verify that the length of the
"renegotiated_connection" field is zero, and if it is not, MUST
abort the handshake (by sending a fatal handshake_failure
alert).
Note: later in Section 3, "abort the handshake" is used as
shorthand for "send a fatal handshake_failure alert and
terminate the connection".
o When the handshake has completed, the client needs to save the
client_verify_data and server_verify_data values for future use.
3.5. Client Behavior: Secure Renegotiation
This text applies if the connection's "secure_renegotiation" flag is
set to TRUE (if it is set to FALSE, see Section 4.2).
o The client MUST include the "renegotiation_info" extension in the
ClientHello, containing the saved client_verify_data. The SCSV
MUST NOT be included.
o When a ServerHello is received, the client MUST verify that the
"renegotiation_info" extension is present; if it is not, the
client MUST abort the handshake.
o The client MUST then verify that the first half of the
"renegotiated_connection" field is equal to the saved
client_verify_data value, and the second half is equal to the
saved server_verify_data value. If they are not, the client MUST
abort the handshake.
o When the handshake has completed, the client needs to save the new
client_verify_data and server_verify_data values.
3.6. Server Behavior: Initial Handshake
Note that this section and Section 3.7 apply to both full handshakes
and session-resumption handshakes.
o When a ClientHello is received, the server MUST check if it
includes the TLS_EMPTY_RENEGOTIATION_INFO_SCSV SCSV. If it does,
set the secure_renegotiation flag to TRUE.
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RFC 5746 TLS Renegotiation Extension February 2010
o The server MUST check if the "renegotiation_info" extension is
included in the ClientHello. If the extension is present, set
secure_renegotiation flag to TRUE. The server MUST then verify
that the length of the "renegotiated_connection" field is zero,
and if it is not, MUST abort the handshake.
o If neither the TLS_EMPTY_RENEGOTIATION_INFO_SCSV SCSV nor the
"renegotiation_info" extension was included, set the
secure_renegotiation flag to FALSE. In this case, some servers
may want to terminate the handshake instead of continuing; see
Section 4.3 for discussion.
o If the secure_renegotiation flag is set to TRUE, the server MUST
include an empty "renegotiation_info" extension in the ServerHello
message.
o When the handshake has completed, the server needs to save the
client_verify_data and server_verify_data values for future use.
TLS servers implementing this specification MUST ignore any unknown
extensions offered by the client and they MUST accept version numbers
higher than their highest version number and negotiate the highest
common version. These two requirements reiterate preexisting
requirements in RFC 5246 and are merely stated here in the interest
of forward compatibility.
Note that sending a "renegotiation_info" extension in response to a
ClientHello containing only the SCSV is an explicit exception to the
prohibition in RFC 5246, Section 7.4.1.4, on the server sending
unsolicited extensions and is only allowed because the client is
signaling its willingness to receive the extension via the
TLS_EMPTY_RENEGOTIATION_INFO_SCSV SCSV. TLS implementations MUST
continue to comply with Section 7.4.1.4 for all other extensions.
3.7. Server Behavior: Secure Renegotiation
This text applies if the connection's "secure_renegotiation" flag is
set to TRUE (if it is set to FALSE, see Section 4.4).
o When a ClientHello is received, the server MUST verify that it
does not contain the TLS_EMPTY_RENEGOTIATION_INFO_SCSV SCSV. If
the SCSV is present, the server MUST abort the handshake.
o The server MUST verify that the "renegotiation_info" extension is
present; if it is not, the server MUST abort the handshake.
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RFC 5746 TLS Renegotiation Extension February 2010
o The server MUST verify that the value of the
"renegotiated_connection" field is equal to the saved
client_verify_data value; if it is not, the server MUST abort the
handshake.
o The server MUST include a "renegotiation_info" extension
containing the saved client_verify_data and server_verify_data in
the ServerHello.
o When the handshake has completed, the server needs to save the new
client_verify_data and server_verify_data values.
4. Backward Compatibility
Existing implementations that do not support this extension are
widely deployed and, in general, must interoperate with newer
implementations that do support it. This section describes
considerations for backward compatible interoperation.
4.1. Client Considerations
If a client offers the "renegotiation_info" extension or the
TLS_EMPTY_RENEGOTIATION_INFO_SCSV SCSV and the server does not reply
with "renegotiation_info" in the ServerHello, then this indicates
that the server does not support secure renegotiation. Because some
attacks (see Section 1) look like a single handshake to the client,
the client cannot determine whether or not the connection is under
attack. Note, however, that merely because the server does not
acknowledge the extension does not mean that it is vulnerable; it
might choose to reject all renegotiations and simply not signal it.
However, it is not possible for the client to determine purely via
TLS mechanisms whether or not this is the case.
If clients wish to ensure that such attacks are impossible, they need
to terminate the connection immediately upon failure to receive the
extension without completing the handshake. Such clients MUST
generate a fatal "handshake_failure" alert prior to terminating the
connection. However, it is expected that many TLS servers that do
not support renegotiation (and thus are not vulnerable) will not
support this extension either, so in general, clients that implement
this behavior will encounter interoperability problems. There is no
set of client behaviors that will guarantee security and achieve
maximum interoperability during the transition period. Clients need
to choose one or the other preference when dealing with potentially
un-upgraded servers.
Rescorla, et al. Standards Track [Page 9]

RFC 5746 TLS Renegotiation Extension February 20104.2. Client Behavior: Legacy (Insecure) Renegotiation
This text applies if the connection's "secure_renegotiation" flag is
set to FALSE.
It is possible that un-upgraded servers will request that the client
renegotiate. It is RECOMMENDED that clients refuse this
renegotiation request. Clients that do so MUST respond to such
requests with a "no_renegotiation" alert (RFC 5246 requires this
alert to be at the "warning" level). It is possible that the
apparently un-upgraded server is in fact an attacker who is then
allowing the client to renegotiate with a different, legitimate,
upgraded server. If clients nevertheless choose to renegotiate, they
MUST behave as described below.
Clients that choose to renegotiate MUST provide either the
TLS_EMPTY_RENEGOTIATION_INFO_SCSV SCSV or "renegotiation_info" in
their ClientHello. In a legitimate renegotiation with an un-upgraded
server, that server should ignore both of these signals. However, if
the server (incorrectly) fails to ignore extensions, sending the
"renegotiation_info" extension may cause a handshake failure. Thus,
it is permitted, though NOT RECOMMENDED, for the client to simply
send the SCSV. This is the only situation in which clients are
permitted to not send the "renegotiation_info" extension in a
ClientHello that is used for renegotiation.
Note that in the case of a downgrade attack, if this is an initial
handshake from the server's perspective, then use of the SCSV from
the client precludes detection of this attack by the server (if this
is a renegotiation from the server's perspective, then it will detect
the attack). However, the attack will be detected by the client when
the server sends an empty "renegotiation_info" extension and the
client is expecting one containing the previous verify_data. By
contrast, if the client sends the "renegotiation_info" extension,
then the server will immediately detect the attack.
When the ServerHello is received, the client MUST verify that it does
not contain the "renegotiation_info" extension. If it does, the
client MUST abort the handshake. (Because the server has already
indicated it does not support secure renegotiation, the only way that
this can happen is if the server is broken or there is an attack.)
4.3. Server Considerations
If the client does not offer the "renegotiation_info" extension or
the TLS_EMPTY_RENEGOTIATION_INFO_SCSV SCSV, then this indicates that
the client does not support secure renegotiation. Although the
attack described in Section 1 looks like two handshakes to the
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RFC 5746 TLS Renegotiation Extension February 2010
server, other attacks may be possible in which the renegotiation is
seen only by the client. If servers wish to ensure that such attacks
are impossible, they need to terminate the connection immediately
upon failure to negotiate the use of secure renegotiation. Servers
that do choose to allow connections from unpatched clients can still
prevent the attack described in Section 1 by refusing to renegotiate
over those connections.
In order to enable clients to probe, even servers that do not support
renegotiation MUST implement the minimal version of the extension
described in this document for initial handshakes, thus signaling
that they have been upgraded.
4.4. Server Behavior: Legacy (Insecure) Renegotiation
This text applies if the connection's "secure_renegotiation" flag is
set to FALSE.
It is RECOMMENDED that servers not permit legacy renegotiation. If
servers nevertheless do permit it, they MUST follow the requirements
in this section.
o When a ClientHello is received, the server MUST verify that it
does not contain the TLS_EMPTY_RENEGOTIATION_INFO_SCSV SCSV. If
the SCSV is present, the server MUST abort the handshake.
o The server MUST verify that the "renegotiation_info" extension is
not present; if it is, the server MUST abort the handshake.
4.5. SSLv3
While SSLv3 is not a protocol under IETF change control (see
[SSLv3]), it was the original basis for TLS and most TLS
implementations also support SSLv3. The IETF encourages SSLv3
implementations to adopt the "renegotiation_info" extension and SCSV
as defined in this document. The semantics of the SCSV and extension
are identical to TLS stacks except for the size of the verify_data
values, which are 36 bytes long each. Note that this will require
adding at least minimal extension processing to such stacks. Clients
that support SSLv3 and offer secure renegotiation (either via SCSV or
"renegotiation_info") MUST accept the "renegotiation_info" extension
from the server, even if the server version is {0x03, 0x00}, and
behave as described in this specification. TLS servers that support
secure renegotiation and support SSLv3 MUST accept SCSV or the
"renegotiation_info" extension and respond as described in this
specification even if the offered client version is {0x03, 0x00}.
SSLv3 does not define the "no_renegotiation" alert (and does
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RFC 5746 TLS Renegotiation Extension February 2010
not offer a way to indicate a refusal to renegotiate at a "warning"
level). SSLv3 clients that refuse renegotiation SHOULD use a fatal
handshake_failure alert.
5. Security Considerations
The extension described in this document prevents an attack on TLS.
If this extension is not used, TLS renegotiation is subject to an
attack in which the attacker can inject their own conversation with
the TLS server as a prefix to the client's conversation. This attack
is invisible to the client and looks like an ordinary renegotiation
to the server. The extension defined in this document allows
renegotiation to be performed safely. Servers SHOULD NOT allow
clients to renegotiate without using this extension. Many servers
can mitigate this attack simply by refusing to renegotiate at all.
While this extension mitigates the man-in-the-middle attack described
in the overview, it does not resolve all possible problems an
application may face if it is unaware of renegotiation. For example,
during renegotiation, either the client or the server can present a
different certificate than was used earlier. This may come as a
surprise to application developers (who might have expected, for
example, that a "getPeerCertificates()" API call returns the same
value if called twice), and might be handled in an insecure way.
TLS implementations SHOULD provide a mechanism to disable and enable
renegotiation.
TLS implementers are encouraged to clearly document how renegotiation
interacts with the APIs offered to applications (for example, which
API calls might return different values on different calls, or which
callbacks might get called multiple times).
To make life simpler for applications that use renegotiation but do
not expect the certificate to change once it has been authenticated,
TLS implementations may also wish to offer the applications the
option to abort the renegotiation if the peer tries to authenticate
with a different certificate and/or different server name (in the
server_name extension) than was used earlier. TLS implementations
may alternatively offer the option to disable renegotiation once the
client certificate has been authenticated. However, enabling these
options by default for all applications could break existing
applications that depend on using renegotiation to change from one
certificate to another. (For example, long-lived TLS connections
could change to a renewed certificate; or renegotiation could select
a different cipher suite that requires using a different
certificate.)
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RFC 5746 TLS Renegotiation Extension February 2010
Finally, designers of applications that depend on renegotiation are
reminded that many TLS APIs represent application data as a simple
octet stream; applications may not be able to determine exactly which
application data octets were received before, during, or after
renegotiation. Especially if the peer presents a different
certificate during renegotiation, care is needed when specifying how
the application should handle the data.
6. IANA Considerations
IANA has added the extension code point 65281 (0xff01), which has
been used for prototype implementations, for the "renegotiation_info"
extension to the TLS ExtensionType values registry.
IANA has added TLS cipher suite number 0x00,0xFF with name
TLS_EMPTY_RENEGOTIATION_INFO_SCSV to the TLS Cipher Suite registry.
7. Acknowledgements
This vulnerability was originally discovered by Marsh Ray and
independently rediscovered by Martin Rex. The general concept behind
the extension described here was independently invented by Steve
Dispensa, Nasko Oskov, and Eric Rescorla with refinements from Nelson
Bolyard, Pasi Eronen, Michael D'Errico, Stephen Farrell, Michael
Gray, David-Sarah Hopwood, Ben Laurie, David Makepeace, Bodo Moeller,
Martin Rex, Peter Robinson, Jesse Walker, Nico Williams, and other
members of the Project Mogul team and the TLS WG.
8. References8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
8.2. Informative References
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
Rescorla, et al. Standards Track [Page 13]